Zr, mo and acid-functional polymer containing aqueous compositions for treatment of metallic surfaces

ABSTRACT

Disclosed herein are a method for treatment of at least one metallic surface of a substrate including at least a step of contacting said surface with an acidic aqueous composition (A), said acidic aqueous composition (A) including Zr ions (a) at least one polymer (P) selected from the group consisting of poly(meth)acrylic acids and (meth)acrylic copolymers, as constituent (b), and Mo ions (c), where the relative weight ratio of Zr ions (a) to Mo ions (c), in each case calculated as metal, is in a range of from 40:1 to 7.5:1, to a corresponding acidic aqueous composition (A) as such,a method of using the acidic aqueous composition (A) for treating metallic surfaces and substrates including the thus treated surfaces.

The present invention relates to a method for treatment of at least one metallic surface of a substrate comprising at least a step of contacting said surface with an acidic aqueous composition (A), said acidic aqueous composition (A) having a pH value in a range of from 2.5 to <5.0 and comprising Zr ions (a) in an amount in a range of from 10 to 200 ppm, calculated as metal, at least one polymer (P) selected from poly(meth)acrylic acids and (meth)acrylic copolymers, which bear carboxylic acid groups, and mixtures thereof, as constituent (b), and Mo ions (c) in an amount in a range of from 0.5 to 10 ppm, calculated as metal, wherein the relative weight ratio of Zr ions (a) to Mo ions (c), in each case calculated as metal, is in a range of from 40:1 to 7.5:1, to a corresponding acidic aqueous composition (A) as such, to the use of the acidic aqueous composition (A) for treating metallic surfaces and to substrates comprising the thus treated surfaces.

BACKGROUND OF THE INVENTION

Aluminum materials made from aluminum and/or an aluminum alloy are typically subjected to an anti-corrosive and adhesion-promoting pretreatment method. Said pretreatment method is generally preceded by pickling the aluminum material. Such pretreatment of aluminum materials is e.g. used for architectural construction elements made of aluminum and/or aluminum alloys in various indoor and outdoor areas, but also e.g. for vehicle parts made of aluminum and/or an aluminum alloy such as wheels. After said pretreatment, usually further coatings are applied to the pretreated aluminum materials.

WO 2010/100187 A1 discloses a two-step method for treatment of metallic surfaces such as surfaces made of aluminum or an aluminum alloy. In a first step the surface is contacted with an aqueous composition containing a silane/silanol/(poly)siloxane. In a subsequent second step the surface is contacted with an aqueous composition containing a phosphonic compound such as a phosphonate/phosphonic acid. Thus, a (poly)siloxane and a phosphonate coating are being successively formed. Such conventional two-step methods in general involve comparably great expenses due to an increased expenditure of time, energy and labor and are therefore disadvantageous.

Conventional aqueous solutions for use in a pretreatment method of aluminum materials are based on complex fluorides such as titanium and/or zirconium complex fluorides in order to form a conversion coating on their surfaces before any further coatings are applied. Subsequently, a further aqueous solution comprising phosphonate compounds may be applied afterwards such that the pretreatment is carried out in form a two-step method. However, the use of such two-step-methods is disadvantageous for the reasons outlined above. Alternatively, the aqueous solutions based on complex fluorides may additionally contain phosphonate compounds such that the pretreatment is performed in total in form of a one-step method. However, the use of phosphonates is undesired for ecological reasons as phosphonates are regarded as contaminants. Due to wastewater regulations and the requirement to purify the wastewater accordingly this is also disadvantageous from an economical view.

The presently known one-step pretreatment methods with complex fluorides such as titanium and/or zirconium complex fluorides, however, do not always deliver satisfying results with respect to a sufficient corrosion protection, in particular regarding the undesired occurrence of filiform corrosion, and/or with respect to sufficient adhesion properties.

For example, in U.S. Pat. no. 4,921,552 a method for coating of aluminum materials or alloys thereof is disclosed. The coating composition used for this purpose inter alia contains a polyacrylic acid polymer and H₂ZrF₆. In U.S. Pat. no. 4,191,596 a further method for coating of aluminum materials or alloys thereof is disclosed. The coating composition used for this purpose inter alia contains a polyacrylic acid polymer or an ester thereof and at least one of H₂TiF₆, H₂ZrF₆, and H₂SiF₆. In addition, WO 97/13588 A1 discloses a method for coating the surface of a metal selected from aluminum and aluminum alloys, which method comprises a step of contacting the surface with an aqueous acid solution containing at least one of H₂TiF₆, H₂ZrF₆, HBF₄ and H₂SiF₆. After a step of rinsing the surface is then further coated with an aqueous polymeric composition. Further, WO 2020/049132 A1, WO 2020/049134 A1 and WO 2019/053023 A1 each relates to a method for treatment of at least one surface of a substrate, wherein said surface is at least partially made of aluminum and/or an aluminum alloy. Each of the methods comprises a step of contacting the surface with an aqueous composition, which contains at least one linear polymer containing inter alia phosphonic acid groups.

Finally, WO 2017/046139 A1 discloses a method for a pretreatment of inter alia workpieces having a surface of aluminum or aluminum alloys, wherein the method inter alia comprises a step of applying an aqueous acidic and chromium-free solution to the workpieces, the solution comprising Zr as complex fluoride, and Mo as molybdate. However, also the solutions disclosed therein do not always deliver satisfying results with respect to a sufficient corrosion protection, in particular regarding the undesired occurrence of filiform corrosion, and/or with respect to sufficient adhesion properties, in particular when powder coating compositions such as (meth)acrylic powder coating compositions are applied subsequently onto the formed conversion coating obtained from using the above mentioned solution.

Thus, there is a need to provide a method for treatment of metallic substrates, in particular at least partially made of aluminum and/or an aluminum alloy, which methods allows formation of a single conversion coating layer in a single step, is both economically and ecologically advantageous, and which provides good anti-corrosion properties as well as no disadvantages with respect to adhesion properties when applying further coatings onto the formed conversion coating layer.

Problem

It has been therefore an object underlying the present invention to provide a method for treatment of metallic substrates, in particular at least partially made of aluminum and/or an aluminum alloy, which methods allows formation of a single conversion coating layer in a single step, and in particular allows avoiding of conventionally used phosphonate treatment steps, is both economically and ecologically advantageous, and which provides good anti-corrosion properties as well as no disadvantages with respect to adhesion properties when applying further coatings onto the formed conversion coating layer.

Solution

This object has been solved by the subject-matter of the claims of the present application as well as by the preferred embodiments thereof disclosed in this specification, i.e. by the subject matter described herein.

A first subject-matter of the present invention is a method for treatment of at least one surface of a substrate, wherein said surface is at least partially made of at least one metal, in particular at least partially made of aluminum and/or an aluminum alloy, comprising at least a step (1), namely

contacting the at least one surface of the substrate with an acidic aqueous composition (A),

-   wherein the acidic aqueous composition (A) has a pH value in a range     of from 2.5 to <5.0 and comprises     -   (a) zirconium ions in an amount in a range of from 10 to 200         ppm, calculated as metal,     -   (b) at least one polymer (P), wherein the polymer (P) is         selected from poly(meth)acrylic acids and (meth)acrylic         copolymers, which bear carboxylic acid groups, and mixtures         thereof, and     -   (c) molybdenum ions in an amount in a range of from 0.5 to 10         ppm, calculated as metal, -   wherein the relative weight ratio of zirconium ions (a) to     molybdenum ions (c), in each case calculated as metal, is in a range     of from 40:1 to 7.5:1.

By contacting step (1) a conversion coating film is formed on the surface of the substrate.

A further subject-matter of the present invention is an acidic aqueous composition (A), said acidic aqueous composition (A) being the one used in the above defined contacting step of the inventive method.

A further subject-matter of the present invention is a use of the inventive acidic aqueous composition (A) for treating at least one surface of a substrate, wherein said surface is at least partially made of at least one metal, preferably at least partially made of aluminum and/or an aluminum alloy, preferably to provide corrosion protection to the surface and/or to the substrate and/or to provide an increased adhesion of a conversion coating formed by the treatment onto the surface to further coatings applied onto the conversion coating, in particular to a further (meth)acrylic based coating such as a (meth)acrylic based powder coating.

A further subject-matter of the present invention is a substrate comprising at least one surface, wherein said surface is at least partially made of at least one metal, preferably at least partially made of aluminum and/or an aluminum alloy, wherein said surface has been treated according to the inventive method and/or by the inventive acidic composition (A).

It has been surprisingly found that due to the presence of the specific combination of inventively used polymer (P) as constituent (b), zirconium ions (a) and molybdenum ions (c) in composition (A) the properties of conversion coatings formed by the contacting step (1), particularly the ability to serve as adhesion promoters for further coatings applied thereon can be significantly improved.

It has been further surprisingly found that due to the presence of the specific combination of inventively used polymer (P) as constituent (b), zirconium ions (a) and molybdenum ions (c) in composition (A) also the corrosive subsurface migration and/or diffusion is/are significantly reduced. It has been in particular found that filiform corrosion is significantly reduced.

Moreover, it has been surprisingly found that the inventive method is economically advantageous as it can be performed in shorter time, energy and labor as the method allows a formation of a single conversion coating layer in a single step. Inparticular, no conventionally used further treatment steps such as a phosphonate treatment step are necessary by using the inventive method. Further, it has been surprisingly found that the inventive method is also ecologically advantageous as no harmful constituents such as chromium containing compounds, in particular Cr(VI) ions, and/or phosphonates have to be present in composition, and that nonetheless excellent adhesion and anti-corrosion properties are obtained.

In addition, it has been found that an undesired yellowing of the conversion coating layer formed can be avoided when using Mo ions in an amount in the range as defined hereinbefore and hereinafter in the aqueous composition and when using a Zr/Mo weight ratio as defined hereinbefore and hereinafter.

DETAILED DESCRIPTION OF THE INVENTION

The term “comprising” in the sense of the present invention, in particular in connection with the inventive method, the inventive(ly used) composition (A) and the inventive master batch, preferably has the meaning “consisting of”. In this case, for example, with regard to inventive composition (A), in addition to the mandatory constituents therein (constituents (a) and (b) and (c) and water) one or more of the other optional constituents mentioned hereinafter may be contained in the composition. All constituents can be present in each case in their preferred embodiments mentioned hereinafter. The same applies to the further subject-matter of the present invention.

Inventive Method

The inventive method is a method for treatment of at least one surface of a substrate, wherein said surface is at least partially made of at least one metal, in particular at least partially made of aluminum and/or an aluminum alloy, comprising at least contacting step (1).

Preferably, the inventive method does not contain any step involving a phosphonate treatment. More preferably, the inventive method does not contain any other step involving any treatment, wherein any further conversion coating film is applied onto the substrate despite the conversion coating film obtained after contacting step (1).

Preferably, the inventive method does not contain any step involving any treatment with chromium ions such as Cr(VI) ions.

Substrate

At least one region of the surface of the substrate is made of at least one metal, preferably made of aluminum and/or of an aluminum alloy. Other examples of metal are different kinds of steel. The surface of the substrate can consist of different regions comprising different metals and/or alloys. However, at least one region of the surface of the substrate is preferably of aluminum and/or an aluminum alloy. Preferably, the overall surface of the substrate is made of aluminum and/or of an aluminum alloy.

More preferably, the substrate as such consists of aluminum and/or of an aluminum alloy, even more preferably of an aluminum alloy.

In case of an aluminum alloy said alloy preferably contains more than 50 wt.-% of aluminum, based on the total weight of the alloy. The method of the invention is in particular suitable for all aluminum alloys containing more than 50 wt.-% aluminum, particularly for aluminum magnesium alloys, including, but not limited to AA5005, as well as for aluminum magnesium silicon alloys, including, but not limited to AA6014, AA6060 and AA6063, for cast alloys - e.g. AlSi7Mg, AlSi9Mg, AISi10Mg, AISi11 Mg, AlSi12Mg - as well as for forge alloys - e.g. AlSiMg. Aluminum magnesium alloys, including AA5005, as well as aluminum magnesium silicon alloys, including AA6060 and AA6063, are commonly used in the field of aluminum finishing and/or for the treatment of wheels and/or in other vehicle parts such as electrical vehicle parts, e.g. battery housings. However, the method is principally suited for all alloys of the so-called AA1000, AA2000, AA3000, AA4000, AA5000, AA6000, AA7000 as well as AA8000 series. A preferred example of the AA2000 series is AA2024. A preferred example of the AA7000 series is AA7075. AA2024 and AA7075 are often used in the aerospace industry.

Most preferred aluminum alloys are selected from the group consisting of aluminum magnesium alloys, aluminum magnesium silicon alloys, aluminum copper alloys, aluminum zinc alloys, and aluminum zinc copper alloys.

The substrates can be wheels or other parts such as automotive parts including vehicle parts in turn including electrical vehicle parts such as battery housings, workpieces and coils. The use of coils is e.g. described in WO 2017/046139 A1. In these cases, the substrate preferably is made of an aluminum magnesium alloy or an aluminum magnesium silicon alloy. The substrates can be parts usable for construction of aeroplanes. In this case, the substrate preferably is made of an aluminum copper alloy or an aluminum zinc alloy.

Contacting Step (1)

Step of the inventive method is a contacting step, wherein the at least one surface of the substrate is contacted with an acidic aqueous composition (A).

The surfaces to be treated may be cleaned by means of an acidic, alkaline or pH-neutral cleaning composition and/or etched before treatment with the acidic aqueous composition (A). The treatment procedure according to step (1), i.e. the “contacting”, can, for example, include a spray coating and/or a dip coating procedure. The composition (A) can also be applied by flooding the surface or by roll coating or even manually by wiping or brushing.

The treatment time, i.e. the period of time the surface is contacted with the acidic aqueous composition (A) used in the method for treatment of a surface according to the invention, is preferably from 15 seconds to 20 minutes, more preferably from 30 seconds to 10 minutes, and most preferably 45 seconds to 5 minutes, as for example 1 to 3 minutes.

The temperature of the acidic aqueous composition (A) used in the inventive method for treatment is preferably from 5 to 50° C., more preferably from 15 to 45° C. and most preferably from 25 to 40° C.

By performing step (1) the inventive method a conversion coating film is formed on the surface of the substrate, which has been in contact with the acidic aqueous composition (A). Preferably, a coating layer is preferably formed after drying that preferably has a coating weight determined by XRF (X-ray fluorescence spectroscopy) of:

0.5 to 100 mg/m², more preferably 0.75 to 50 mg/m², even more preferably 1 to 40 mg/m², still more preferably 2 to 35 mg/m², yet more preferably 5 to 30 mg/m², in particular 10 to 26 mg/m², of zirconium and molybdenum ions used as constituents (a) and (c), each calculated as metal.

Optional Further Steps of the Inventive Method

Prior to step (1) one or more of the following optional steps can be performed in this order:

-   Step (A-1): cleaning and optionally subsequently rinsing the surface     of the substrate, -   Step (B-1): subjecting the surface of the substrate to acidic     pickling, i.e., etching, and subsequently rinsing the surface of the     substrate, -   Step (C-1): contacting the surface of the substrate with an aqueous     composition comprising at least one mineral acid, said aqueous     composition being different from composition (A) or alternatively     with an aqueous alkaline composition or pH-neutral aqueous     composition and -   Step (D-1): rinsing the surface of the substrate obtained after the     contact according to step (C-1) and/or (B-1).

Alternatively, steps (A-1) and (B-1) may be performed in one step, which is preferred. Preferably, both steps (A-1) and (B-1) are performed.

Optional step (C-1) serves to remove aluminum oxide, undesired alloy components, the skin, brushing dust etc. from the surface of the substrate and to thereby activate the surface for the subsequent conversion treatment in step (1) of the method according to the invention. This step represents an etching step.

Preferably, the at least one mineral acid of the composition in step (C-1) is sulfuric acid and/or nitric acid, more preferably sulfuric acid. The content of the at least one mineral acid is preferably in the range of 1.5 to 75 g/l, more preferably of 2 to 60 g/l and most preferably of 3 to 55 g/l. The composition used in step (C-1) preferably additionally comprises one or more metal ions selected from the group of titanium, zirconium, hafnium ions and mixtures thereof. In the treatment of parts, the duration of treatment with the composition in step (C-1) is preferably in the range of 30 seconds to 10 minutes, more preferably of 40 seconds to 6 minutes and most preferably of 45 seconds to 4 minutes. The treatment temperature is preferably in the range of 20 to 55° C., more preferably of 25 to 50° C. and most preferably of 30 to 45° C. In the treatment of coils, the duration of treatment is preferably in the range of 3 seconds to 1 minute, most preferably of 5 to 20 seconds.

Rinsing step (D-1) and the optional rinsing being part of step (A-1) are preferably performed by using deionized water or tap water. Preferably, step (D-1) is performed by using deionized water.

After having performed mandatory step (1) of the inventive method one or more of the following optional steps can be performed in this order:

-   Step (2): rinsing the surface of the substrate obtained after the     contact according to step (1), -   Step (3): contacting the surface of the substrate obtained after     step (1) or after optional step (2) with an aqueous acidic     composition (B) being the same or different from composition (A), -   Step (4): rinsing the surface of the substrate obtained after the     contact according to step (3), and -   Step (5): drying the surface of the substrate obtained after the     contact according to step (1), after the rinsing of step (2), after     the contact according to step (3) or after the rinsing of step (4).

After step (1) of the method according to the invention the surface of the substrate obtained after contact according to step (1) can be rinsed, preferably with deionized water or tap water (optional step (2)). After optional step (3) of the method according to the invention the surface of the substrate obtained after contact according to optional step (3) can be rinsed, preferably with water (optional step (4)).

Rinsing steps (2) and (4) may be carried out in order to remove excess components present in composition (A) used in step (1) and optionally also in the composition used in optional step (3) such as for example the polymer (P) and/or disruptive ions from the substrate.

In one preferred embodiment, rinsing step (2) is carried out after step (1). In another preferred embodiment, no rinsing step (2) is performed. In both embodiments, an additional drying step (5) is preferably performed. By drying step (5) at least the conversion coating film present on the surface of the substrate is dried and becomes a coating layer.

The aqueous composition (B) applied in step optional step (3) of the method according to the invention may for example be another composition as used in step (1), i.e. a composition, which is different from the composition (A) used in step (1), but does not necessarily have to, i.e. can be identical to composition (A).

The surfaces of the inventively used substrate can be coated by further, i.e. subsequent coatings. The inventive method thus may contain at least one further optional step, namely

Step : applying at least one coating composition to the surface of the substrate obtained after step (1) or after any of optional steps (2) to (5) to form a coating film upon the surface, said coating film being different from the conversion coating film obtained after step (1).

The coating composition used in step (6) is different from compositions (A) and (B) and preferably comprises at least one polymer being suitable as binder, said polymer being different from polymer (P). Examples of such polymers being different from polymer (P) are in particular polyesters, polyurethanes, epoxy-based polymers (epoxy resins) and/or (meth)acrylic copolymers and/or polyvinylidene fluorides (PVDF). If applicable, these polymers are used in combination with crosslinking agents such as blocked polyisocyanates and/or aminoplast resins.

Preferably, step (6) is performed. The coating composition used in step (6) can be a powder coating composition. Alternatively, it can be a solvent-borne or aqueous coating composition. Preferably, a powder coating composition is used. Any conventional powder coating composition may be used in such a step. The coating composition used can be in particular a primer coating composition or a clearcoat composition.

Preferably, the inventive method comprises said step (6) as an additional coating step of applying at least one coating composition to the surface of the substrate obtained after the contacting step (1) - i.e. to the surface of the substrate bearing a conversion coating layer due to having performed step (1), to form at least one further coating layer upon the surface, wherein optionally after step (1) a rinsing step (2) is carried out prior to said coating step (6). Independently whether said optional rinsing step (2) is performed or not, a drying step (5) is preferably carried out in turn prior to coating step (6).

Before the application of further coatings according to step (6) the treated surface is preferably rinsed to remove excessive polymer (P) as well as optionally present unwanted ions.

The subsequent coatings can be applied wet-on-wet onto the metallic surface as treated in the method for treatment according to the invention. However, it is also possible to dry the metallic surface as treated according to the invention in step (5) before applying any further coating.

Composition (A) Used in Step (1) of the Inventive Method

The acidic aqueous composition (A) used in step (1) is preferably free of any chromium ions such as Cr(VI) cations.

The acidic aqueous composition (A) used in step (1) is preferably free of any phosphonate anions.

The term “aqueous” with respect to the inventively used composition (A) in the sense of the present invention preferably means that the composition (A) is a composition containing at least 50 wt.-%, preferably at least 60 wt.-%, more preferably at least 70 wt.-% in particular at least 80 wt.-%, most preferably at least 90 wt.-% of water, based on its total content of organic and inorganic solvents including water. Thus, the composition (A) may contain at least one organic solvent besides water - however, in an amount lower than the amount of water present.

Preferably, the acidic aqueous composition (A) contains at least 50 wt.-%, preferably at least 60 wt.-%, more preferably at least 70 wt.-% in particular at least 80 wt.-%, most preferably at least 90 wt.-% of water, in each case based on its total weight.

The acidic aqueous composition (A) has a pH value in a range of from 2.5 to <5.0. Preferably, the pH value is measured at room temperature (23° C.). The pH value of the acidic aqueous composition is preferably in the range in the range of from 2.6 to 4.8, preferably of from 2.8 to 4.6, more preferably of from 3.0 to 4.4, even more preferably of from 3.1 to 4.3, in particular of from 3.2 to 4.2, most preferably of from 3.2 to 3.8. The pH can be preferably adjusted by using nitric acid, aqueous ammonia and/or sodium carbonate if necessary.

The acidic aqueous composition (A) can be used as a dip coat bath. However, it can also be applied by virtually any conventional coating procedure like e.g. spray coating, roll coating, brushing, wiping etc. as outlined above in connection with step (1). Spraying is preferred.

The inventively used acidic aqueous composition (A) may comprise further components including ions as lined out in the detailed description hereinafter. The term “further comprises”, as used herein throughout the description in view of the ingredients of acidic aqueous compositions, means “in addition to the mandatory constituents (a) and (b) and (c) as well as water. Therefore, such “further” compounds including ions differ from the mandatory ingredients (a) and (b) and (c).

The terms “constituents” and “components” used herein are inter-changeable.

The total amount of all components (constituents) present in the inventive composition (A) adds up to 100 wt.-%.

Composition (A) can be a dispersion or solution. Preferably, it is a solution.

Preferably, the acidic aqueous composition (A) has a temperature in a range of from 18 to 35° C., more preferably of from 20 to 35° C., in particular of from 20 to 30° C.

Preferably, the acidic aqueous composition (A) comprises

-   zirconium ions as constituent (a) in an amount in a range of from 15     to 150 ppm, more preferably of from 16 to 125 ppm, yet more     preferably of from 17 to 100 ppm, even more preferably of from 18 to     75 ppm, still more preferably of from 20 to 65 ppm, yet more     preferably of from 20 to 35 ppm, in each case calculated as metal,     and/or, preferably and, -   molybdenum ions in an amount in a range of from 1 to 8 ppm, more     preferably of from 1.5 to 6 ppm, yet more preferably of from 1.5 to     5 ppm, even more preferably of from 1.5 to 4 ppm, still more     preferably of from 2.0 to 3.5 ppm, yet more preferably of from >2 to     3 ppm, in each case calculated as metal.

The relative weight ratio of zirconium ions (a) to molybdenum ions (c) in composition (A) is, in each case calculated as metal, in a range of from 40:1 to 7.5:1.

Preferably, the relative weight ratio of zirconium ions (a) to molybdenum ions (c), in each case calculated as metal, is in a range of from 35:1 to 8:1, more preferably of from 25:1 to 8.5:1, yet more preferably of from 15:1 to 9:1, even more preferably of from 12.5:1 to 9.5:1, yet more preferably of from 12:5 to 10:1.

Zirconium Ions as Constituent (a)

Composition (A) contains zirconium ions in an amount in a range of from 10 to 200 ppm, calculated as metal.

Preferably, the acidic aqueous composition (A) comprises zirconium ions in an amount in a range of from 15 to 150 ppm, more preferably of from 16 to 125 ppm, yet more preferably of from 17 to 100 ppm, even more preferably of from 18 to 75 ppm, still more preferably of from 20 to 65 ppm, yet more preferably of from 20 to 35 ppm, in each case calculated as metal.

Preferably, the amount of component (a) in ppm in the composition (A) is lower than the amount of component (b) in ppm.

Preferably, a precursor metal compound is used to generate the ions as constituent (a) in composition (A). Preferably, the precursor metal compound is water-soluble. Solubility is determined at a temperature of 20° C. and atmospheric pressure (1.013 bar).

The content of component (a) can be monitored and determined by the means of ICP-OES (optical emission spectroscopy with inductively coupled plasma). Said method is described hereinafter in detail.

Particularly preferred zirconium compounds are the complex fluorides of these metals. The term “complex fluoride” includes the single and multiple protonated forms as well as the deprotonated forms. It is also possible to use mixtures of such complex fluorides. Complex fluorides in the sense of the present invention are complexes of zirconium formed with fluoride ions in composition (A), e.g. by coordination of fluoride anions to zirconium cations in the presence of water.

Moreover, zirconium can also be added in form of zirconyl compounds as e.g. zirconyl nitrate and zirconyl acetate; or zirconium carbonate or zirconium nitrate, the latter one being particularly preferred.

However, preferably, the zirconium ions (a) are incorporated into composition (A) in form of their complex fluorides.

Polymer (P) as Constituent (b)

Composition (A) contains at least one polymer (P), is wherein the polymer (P) is selected from poly(meth)acrylic acids and (meth)acrylic copolymers, which bear carboxylic acid groups, and mixtures thereof. Thus, polymer (P) is selected from poly(meth)acrylic acids (bearing carboxylic acid groups as it is evident from the term “poly(meth)acrylic acids” per se), from carboxylic acid groups containing (meth)acrylic copolymers, and mixtures thereof.

Polymer (P) is preferably soluble in acidic composition (A). Solubility is determined at a temperature of 20° C. and atmospheric pressure (1.013 bar).

Polymer (P) is preferably present in composition (A) in an amount in the range of from 50 to 2000 ppm, preferably in the range of from 60 to 1500 ppm, more preferably in the range of from 70 to 1000 ppm, even more preferably in the range of from 80 to 750 ppm, yet more preferably in the range of from 90 to 650 ppm, still more preferably of from 100 to 600 ppm, even more preferably of from 105 to 500 ppm, yet more preferably of from 125 to 400 ppm, yet more preferably of from 135 to 300 ppm, most preferably of from 150 to 250 ppm.

Preferably, polymer (P) does not contain any phosphonic acid and/or phosphonate groups.

Preferably, the at least one polymer (P) has a weight average molecular weight (M_(w)) in a range of from 40000 to 350000 g/mol, preferably of from 50000 to 340000 g/mol, more preferably of from 60000 to 330000 g/mol, still more preferably of from 65000 to 320000 g/mol, yet more preferably of from 70000 to 310000 g/mol, still more preferably of from 70000 to 300000, yet more preferably of from 75000 to 275000 g/mol. The weight average molecular weight is determined by the method described hereinafter in the ‘methods’ section.

Preferably, the polydispersity of polymer (P) exceeds a value of 2.0, more preferably exceeds a value of 2.5. Preferably, the polydispersity is in a range of from >1.0 to 4.0. The polydispersity is determined by the method described hereinafter in the ‘methods’ section.

Polymer (P) bears carboxylic acid groups. Polymer (P) is selected from poly(meth)acrylic acids and (meth)acrylic copolymers, which bear carboxylic acid groups, and mixtures thereof. Preferably, polymer (P) is a poly(meth)acrylic acid, more preferably a polyacrylic acid.

In any case, polymer (P) is prepared from making use of (meth)acrylic monomers.

The term “(meth)acryl” means “acryl” and/or “methacryl”. Similarly, “(meth)acrylate” means acrylate and/or methacrylate. Polymer (P) is preferably a “(meth)acryl polymer”, which is formed from “acryl monomers” and/or “methacryl monomers”, but additionally may contain non-acryl and non-methacryl monomeric units if other ethylenically unsaturated monomers such as vinyl monomers are additionally used in case polymer (P) is a copolymer. Preferably, the backbone of the (meth)acryl copolymer (P) is formed from more than 50 mol-%, even more preferably of from more than 75 mol-%, of (meth)acryl monomers.

Preferably, polymer (P) is a (meth)acrylic copolymer and comprises a polymeric backbone and at least one kind of side chains attached to said polymeric backbone, which bear carboxylic acid groups.

Polymer (P) preferably is a linear polymer. In case polymer (P) is a copolymer the monomeric units present in polymer (P) can be arranged statistically, in two or more blocks or as a gradient along the polymeric backbone of polymer (P). Such arrangements can also be combined. Preferably, polymer (P) has a statistical distribution in case it is a copolymer and can be prepared by conventional radical polymerization. If polymer (P) is a block copolymer it can be preferably prepared by controlled radical polymerization.

Preferably, polymer (P), contains monomeric units (s1) present in the polymer, which each contain a side chain (S1) comprising at least one carboxylic acid group in an amount of 50 to 100 mol-%, more preferably of 75 to 100 mol-%, even more preferably of 90 to 100 mol-%, in particular of 100 mol-%, in each case based on the total amount of all monomeric units of polymer (P), wherein the sum of all monomeric units present in polymer (P) adds up to 100 mol-%. In case of 100 mol-% polymer (P) is a homopolymer composed in total of monomeric units (s1) present in the polymer, which each contain a side chain (S1) comprising at least one carboxylic acid group, which is most preferred.

The functional groups of side chains (S1) not only allow crosslinking reactions to take place when a further coating film is applied on top of the conversion coating film obtained after performing step (1) of the inventive method, when the coating composition used for forming the further coating film comprises suitable film-forming polymers and/or crosslinking agent having in turn functional groups that are reactive towards the functional groups of side chain (S1), but the functional groups of side chains (S1) additionally are relevant in order to ensure that polymer (P) has a sufficient solubility in water and thus in the aqueous composition (A).

Preferably, the at least one monomer used for preparing polymer (P) is selected from the group consisting of preferably (meth)acrylic monomers having at least one COOH-group. Examples are acrylic acid and methacrylic acid. Alternatively or additionally, other carboxylic acid group containing monomers may be used for its preparation such as maleic acid and/or maleic acid anhydride, in particular when polymer (P) is a copolymer.

The polymer (P) may additionally contain further monomeric units different from (m1) in case it is a copolymer. These may contain side chains containing in turn OH-groups. Examples of suitable monomers for introducing such side chains are 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 3-hydroxypropyl acrylate, 3-hydroxypropyl methacrylate, 4-hydroxybutyl acrylate, 4-hydroxybutyl methacrylate, 3-phenoxy-2-hydroxypropyl (meth)acrylate, glycerol mono (meth)acrylate, N-(2-hydroxypropyl) (meth)acrylamide, allyl alcohol, hydroxystyrene, hydroxyalkyl vinyl ethers such as hydroxybutyl vinyl ether and vinylbenzyl alcohol.

Molybdenum Ions as Constituent (c)

Composition (A) contains molybdenum ions in an amount in a range of from 0.5 to 10 ppm, calculated as metal.

Preferably, the acidic aqueous composition (A) comprises molybdenum ions in an amount in a range of from 1 to 8 ppm, more preferably of from 1.5 to 6 ppm, yet more preferably of from 1.5 to 5 ppm, even more preferably of from 1.5 to 4 ppm, still more preferably of from 2.0 to 3.5 ppm, in particular of from >2 to 3 ppm, in each case calculated as metal.

For preparing the aqueous composition (A) preferably a water-soluble (at a temperature of 20° C. and atmospheric pressure (1.013 bar)) molybdenum salt is used. Preferably, the molybdenum ions (c) are incorporated into composition (A) in form of at least one molybdate, preferably at least one ammonium molybdate.

Further Optional Constituents

The inventive(ly) used acidic aqueous composition (A) preferably contains free fluorides. These may result from the presence of component (a), i.e. in particular when complex fluorides of Zr are present in (A) as component (a), but may also or alternatively result from the presence of other optional components as described hereinafter. Preferably, the acidic aqueous composition (A) contains free fluoride ions in an amount in the range of from 1 to 500 ppm, more preferably of from 1.5 to 200 ppm, even more preferably of from 2 to 100 ppm, in particular of from 2.5 to 50 ppm. The free fluoride content is determined by means of a fluoride ion sensitive electrode according to the method disclosed in the ‘methods’ section.

Optionally, aqueous composition (A) further comprises at least one kind of metal cations selected from the group of cations of metals of the 1^(st) to 3^(rd) subgroup (copper, zinc and scandium groups) and 4^(th) to 8^(th) subgroup (titanium, vanadium, chromium, manganese, iron, cobalt and nickel groups) of the periodic table of the elements including the lanthanides as well as the 2^(nd) main group of the periodic table of the elements (alkaline earth metal group), lithium, bismuth and tin. The beforementioned metal cations are generally introduced in form of their water-soluble compounds, preferably as their water-soluble salts. Preferred cation(s) is/are selected from the group consisting of cations of cerium and the other lanthanides, chromium, iron, calcium, cobalt, copper, magnesium, manganese, nickel, niobium, tantalum, yttrium, vanadium, lithium, titanium, hafnium, bismuth, zinc and tin. Such metal cations are different from constituents (a) and (c).

Optionally, aqueous composition (A) further comprises at least one pH-Value adjusting substances, preferably selected from the group consisting of nitric acid, sulfuric acid, methanesulfonic acid, acetic acid, aqueous ammonia, sodium hydroxide and sodium carbonate, wherein nitric acid, aqueous ammonia and sodium carbonate are preferred. Depending on the pH value of the acidic aqueous composition (A), the above compounds can be in their fully or partially deprotonated form or in protonated forms.

Optionally, aqueous composition (A) further comprises at least one water-soluble fluorine compound. Examples of such water-soluble fluorine compounds are fluorides as well as hydrofluoric acid. In particular, such a compound is present in composition (A), when component (a) is not present in the form of a complex fluoride of zirconium in composition (A).

Optionally, aqueous composition (A) further comprises at least one corrosion inhibitor. Examples are L-cysteine and other amino acids, benzotriazole and mixtures thereof. Preferably, the at least one corrosion inhibitor does not comprise any kind of metal ions.

Inventive Composition (A)

A further subject-matter of the present invention is an acidic aqueous composition (A), said acidic aqueous composition (A) being the one used in the above defined contacting step (1) of the inventive method.

All preferred embodiments described above herein in connection with the inventive method and the inventively used composition (A), which is used in the contacting step (1) of said method, and the constituents contained therein, in particular components (a), (b) and water, but also optional components are also preferred embodiments of inventive acidic aqueous composition (A) as such.

Inventive Master Batch

A further subject-matter of the present invention is a master batch to produce the inventive acidic aqueous composition (A) by diluting the master batch with water and if applicable by adjusting the pH value.

All preferred embodiments described above herein in connection with the inventive method and the inventive composition (A), which is used in the contacting step (1) of said method, and the constituents contained therein, in particular components (a) and (b) and (c) besides water, but also optional components as well as described above herein in connection with acidic aqueous composition (A) as such are also preferred embodiments of the inventive master batch.

If a master batch is used to produce the acidic aqueous composition (A) according to the present invention, the master batch typically contains the ingredients of the acidic aqueous composition (A) to be produced in the desired proportions, namely constituents (a) and (b) and (c), but at a higher concentration. Such master batch is preferably diluted with water to the concentrations of ingredients as disclosed above to form the acidic aqueous composition (A). If necessary, the pH value of the acidic aqueous composition may be adjusted after dilution of the master batch.

Of course, it is also possible to further add any of the optional constituents to the water, wherein the master batch is diluted or to add any of the optional constituents after diluting the master batch with water. It is however preferred that the master batch already contains all necessary constituents.

Preferably, the master batch is diluted with water and/or an aqueous solution in the ratio of 1:5,000 to 1:10, more preferred 1:1,000 to 1:10, most preferred in the ratio of 1:300 to 1:10 and even more preferred 1:150 to 1:50.

Inventive Use

A further subject-matter of the present invention is a use of the inventive acidic aqueous composition (A) for treating at least one surface of a substrate, wherein said surface is at least partially made of at least one metal, preferably at least partially made of aluminum and/or an aluminum alloy, preferably to provide corrosion protection to the surface and/or to the substrate and/or to provide an increased adhesion of a conversion coating formed by the treatment onto the surface to further coatings applied onto the conversion coating.

All preferred embodiments described above herein in connection with the inventive method and the inventive composition (A), which is used in the contacting step (1) of said method, as well as the inventive master batch, and the constituents contained therein and in the composition, in particular components (a) and (b) and (c) besides water, but also optional components, as well as described above herein in connection with acidic aqueous composition (A) as such are also preferred embodiments of the inventive use.

Inventive Substrate

A further subject-matter of the present invention is a substrate comprising at least one surface, wherein said surface is at least partially made of at least one metal, preferably at least partially made of aluminum and/or an aluminum alloy, wherein said surface has been treated according to the inventive method and/or by the inventive acidic composition (A). By the inventive treatment a conversion coating film is formed and thus present on the substrate. Thus, the inventive substrate represents a coated substrate.

All preferred embodiments described above herein in connection with the inventive method and the inventive composition (A), which is used in the contacting step (1) of said method, as well as the inventive master batch, and the constituents contained therein and in the composition, in particular components (a) and (b) and (c) besides water, but also optional components, as well as described above herein in connection with acidic aqueous composition (A) as such, and the inventive use, are also preferred embodiments of the inventive substrate.

In particular the coated substrates bears a coating layer having a coating weight after drying determined by XRF (X-ray fluorescence spectroscopy) of 0.5 to 100 mg/m², more preferably 0.75 to 50 mg/m², even more preferably 1 to 40 mg/m², still more preferably 2 to 35 mg/m², yet more preferably 5 to 30 mg/m², in particular 10 to 26 mg/m²,of zirconium and molybdenum ions used as constituents (a) and (c), each calculated as metal.

METHODS 1. Determination of Average Molecular Weights M_(w) and M_(n)

The number average and weight average molecular weights (M_(n) and M_(w)), respectively, are measured according to the following protocol: Samples are analyzed by SEC (size exclusion chromatography) equipped with a MALS detector. Absolute molar masses are obtained with a dn/dC value chosen equal to 0.1875 mL/g in order to get a recovery mass around 90%. Polymer samples are dissolved in the mobile phase and the resulting solutions are filtrated with a Millipore filter 0.45 µm. Eluting conditions are the following ones. Mobile phase: H₂O 100% vol. 0.1 M NaCl, 25 mM NaH₂PO₄, 25 mM Na₂HPO₄; 100 ppm NaN₃; flow rate: 1 mL/min; columns: Varian Aquagel OH mixed H, 8 µm, 3*30 cm; detection: RI (concentration detector Agilent) + MALLS (Multi Angle Laser Light Scattering) Mini Dawn Tristar + UV at 290 nm; samples concentration: around 0.5 wt% in the mobile phase; injection loop: 100 µL. Polydispersity P can be calculated from the M_(n) and M_(w) values obtained.

2. Free Fluoride Content Determination

The free fluoride content is determined by means of a fluoride ion selective electrode. The electrode is calibrated using at least three master solutions with known fluoride concentrations. The calibration process results in the building of calibration curve. Then the fluoride content is determined by using of the curve.

3. Icp-oes

The amount of certain elements in a sample under analysis, such as of zirconium and molybdenum, being present as components (a) and (c), is determined using inductively coupled plasma atomic emission spectrometry (ICP-OES) according to DIN EN ISO 11885 (date: Sep. 1, 2009). A sample is subjected to thermal excitation in an argon plasma generated by a high-frequency field, and the light emitted due to electron transitions becomes visible as a spectral line of the corresponding wavelength and is analyzed using an optical system. There is a linear relation between the intensity of the light emitted and the concentration of the element in question. Prior to implementation, using known element standards (reference standards), the calibration measurements are carried out as a function of the particular sample under analysis. These calibrations can be used to determine concentrations of unknown solutions such as the concentration of the amount of titanium, zirconium and hafnium.

4. Crosscut Testing to DIN EN ISO 2409 (06-2013)

The crosscut test is used to ascertain the strength of adhesion of a coating on a substrate in accordance with DIN EN ISO 2409 (06-2013). Cutter spacing is 2 mm. Assessment takes place on the basis of characteristic cross-cut values in the range from 0 (very good adhesion) to 5 (very poor adhesion). This method is used for measurement of the dry adhesion. The crosscut test is also performed after storing the sample for 48 h in water having a temperature of 63° C. in order to determine the wet adhesion. The crosscut test may also be performed after exposure for up to 240 hours in a condensation climate test according to DIN EN ISO 6270-2 CH (09-2005 and the correction of 10-2007). Each of the tests is performed three times and an average value is determined.

5. Filiform Corrosion (FFC)

Determining the filiform corrosion is used to ascertain the corrosion resistance of a coating on a substrate. This determination is carried out according to MBN 10494-6, 5.5 (Edition 2016-03) over a duration of 672 hours. The maximum thread length (LF) and/or the average filiform subsurface corrosion (UF) in [mm] is measured.

6. Copper Catalyzed Acetic Acid Salt Spray (CASS) Testing to DIN EN ISO 9227 (09-2012)

The copper catalyzed acetic acid salt spray fog test is used for determining the corrosion resistance of a coating on a substrate. In accordance with DIN EN ISO 9227 (09-2012) the samples under analysis are in a chamber in which there is continuous misting of a 5% strength common salt solution, the salt solution being admixed with acetic acid and copper chloride, at a temperature of 50° C. over a certain period of time such as over a duration of 168, 240 or 264 hours, respectively, with controlled pH. The spray mist deposits on the samples under analysis, covering them with a corrosive film of salt water. If, still prior to the CASS testing, the coating on the samples for investigation is scored down to the substrate with an incision, the samples can be investigated for their level of under-film corrosion in accordance with DIN EN ISO 4628-8 (03-2013), since the substrate corrodes along the score line during the CASS test. As a result of the progressive process of corrosion, the coating is under-rusted to a greater or lesser extent during the test. The extent of under-rust in [mm] is a measure of the resistance of the coating. Each of the tests is performed three times and an average value is determined. Both an average value (mean value) for the corrosion (“c″-value”) and delamination (“d″-value) can be determined.

EXAMPLES

The following examples further illustrate the invention, but are not to be construed as limiting its scope.

1. Preparation of Acidic Aqueous Compositions

1.1 A number of acidic aqueous compositions have been prepared (1 L each). All aqueous compositions contained H₂ZrF₆ in an amount that corresponds to the ppm values of zirconium, calculated as metal, as illustrated in Table 1 below. All aqueous compositions further contained ammonium heptamolybdate in an amount that corresponds to the ppm values of molybdenum, calculated as metal, as illustrated in Table 1 below. Each of the compositions further contained Sokolan® PA 110S as polymer, which is a commercially available polyacrylic acid having a M_(w) of 250,000 g/mol.

In Table 1a the acidic aqueous compositions that have been prepared in this manner are summarized.

TABLE 1a Composition (all inventive) pH value Amount of polymer [ppm] Amount of Zr [ppm] Amount of Mo [ppm] Zr:Mo-weight ratio A1 3.6 175 23 2.4 9.58:1 A2 4.2 175 21 2.0 10.5:1 A3 4.1 175 63 2.0 31.5:1

1.2 A number of further acidic aqueous compositions have been prepared (1 L each) in the same as described in item 1.1 by using the same ingredients. However, different amounts/pH values were used.

In Table 1b the acidic aqueous compositions that have been prepared in this manner are summarized.

TABLE 1b Composition pH value Amount of polymer [ppm] Amount of Zr [ppm] Amount of Mo [ppm] Zr:Mo-weight ratio A4 (comparative) 3.2 175 12 24 1:2 A5 (inventive) 3.4 175 23 2.4 9.58:1

2. Pretreatment Method

As substrate an aluminum alloy substrate (substrate T1; AA5005) has been used. A5505 is an aluminum magnesium alloy substrate.

The substrate was cleaned by making use of the commercial product Gardoclean® S 5201/1 (3 minutes at 63° C.). Then, rinsing with tap water was performed twice (for 30 seconds each). Next, an etching step was performed. The etching was performed by making use of a mixture of the commercial product Gardacid® 4325 (containing nitric acid; 50 g/L; Chemetall GmbH) and of the commercial product Gardobond® Additive H 7274 (containing fluoride; 7.5 g/L; Chemetall GmbH) (60 seconds). After carrying out the etching a rinsing with tap water (30 seconds) followed by rinsing with deionized water (30 seconds) was performed.

Then a contacting step was carried out, i.e. the surface of the substrate was contacted with one of the acidic aqueous compositions described hereinbefore in items 1.1 and 1.2 in order to form a conversion coating layer on the surface of the substrate. The contacting step was performed in each case for 60 seconds by spraying of one of the acidic aqueous compositions onto the surfaces of the substrates. The acidic aqueous compositions were heated to 25° C. before spraying. As a reference example (RE) a conventional two-step contacting pretreatment was performed: an aqueous commercially available composition (Gardobond® X 4707) not containing any polymer has been used in a first contacting step and then a second contacting step with a commercially available aqueous phosphonate containing solution (Gardobond® X 4661) has been used after rinsing.

Following the contacting step a drying step is performed (15 minutes at 60 to 70° C.) after a period of air blowing.

Afterwards, a coating layer was applied onto the conversion-coated substrate T1. An acrylic coating material was used, namely a commercially available acrylic power coating material (PY1005 from FreiLacke). The dry layer thicknesses of these coatings obtained were in the range of from 80 to 100 µm.

3. Properties of the Coated Substrates

A number of properties of the coated substrates obtained by the method described hereinbefore in item 2. have been investigated. These properties were determined according to the test methods described hereinbefore. The results are displayed in Table 2a and Table 2b. In addition, the coating weights have been measured by XRF.

TABLE 2a Substrate T1 Aqueous composition used for pretreatment Crosscut after condensation climate test for 240 h Filiform corrosion (LF) Coating weight Zr [mg/m²] Coating weight Mo [mg/m²] RE (reference) 2 5.0 3 - A1 1 3.0 14 10 A2 0 3.5 26 14 A3 0 3.5 23 14

As it is evident from Table 2a excellent adhesion and anti-corrosion properties were obtained when using a one-step treatment method and making use of an inventive aqueous composition. The adhesion and anti-corrosion properties were significantly better compared to reference example RE.

TABLE 2b Substrate T1 Aqueous composition used for pretreatment Crosscut after condensation climate test for 240 h Under-rust after CASS test [mm] Coating weight Zr [mg/m²] Coating weight Mo [mg/m²] A4 (comparative) 2 to 3 10 to 12 <1 40 A5 0 to 1 0.8 to 1 14 10

As it is evident from Table 2b excellent adhesion and anti-corrosion properties were obtained when using a one-step treatment method and making use of an inventive aqueous composition. The adhesion and anti-corrosion properties were significantly better when having utilized composition A5 compared to having performed the method with comparative example A4 having both a content of Mo ions, which is too high and a Zr/Mo weight ratio outside a range of from 40:1 to 7.5:1. In addition, in case of having used composition A4 an undesired yellowish color of the resulting coating layer has been observed. It has been found that this undesired yellowing is observed when the amount of Mo ions is too high/the above mentioned Zr/Mo weight ratio is not met. 

1. A method for treatment of at least one surface of a substrate, wherein said surface is at least partially made of at least one metal, comprising (1) contacting the at least one surface of the substrate with an acidic aqueous composition (A), wherein the acidic aqueous composition (A) has a pH value in a range of from 2.5 to <5.0 and comprises (a) zirconium ions in an amount in a range of from 10 to 200 ppm, calculated as metal, (b) at least one polymer (P), wherein the polymer (P) is selected from the group consisting of poly(meth)acrylic acids and (meth)acrylic copolymers, which bear carboxylic acid groups, and mixtures thereof, and (c) molybdenum ions in an amount in a range of from 0.5 to 10 ppm, calculated as metal, wherein the relative weight ratio of zirconium ions (a) to molybdenum ions (c), in each case calculated as metal, is in a range of from 40:1 to 7.5:1.
 2. The method according to claim 1, wherein the relative weight ratio of zirconium ions (a) to molybdenum ions (c), in each case calculated as metal, is in a range of from 35:1 to 8:1.
 3. The method according to claim 1, wherein the acidic aqueous composition (A) comprises zirconium ions in an amount in a range of from 15 to 150 ppm, in each case calculated as metal, and/or, molybdenum ions in an amount in a range of from 1 to 8 ppm in each case calculated as metal.
 4. The method according to claim 1, wherein the acidic aqueous composition (A) has a pH value in a range of from 2.6 to 4.8.
 5. The method according to claim 1, wherein the acidic aqueous composition (A) has a temperature in a range of from 18 to 35° C.
 6. The method according to claim 1, wherein the at least one polymer (P) has a weight average molecular weight (M_(w)) in a range of from 40000 to 350000 g/mol.
 7. The method according to claim 1, wherein polymer (P) is a poly(meth)acrylic acid .
 8. The method according to claim 1, wherein polymer (P) is present in composition (A) in an amount in the range of from 50 to 2000 ppm.
 9. The method according to claim 1, wherein the zirconium ions (a) are incorporated into composition (A) in form of their complex fluorides.
 10. The method according to claim 1, wherein the molybdenum ions (c) are incorporated into composition (A) in form of at least one molybdate .
 11. The method according to claim 1, wherein the acidic aqueous composition (A) contains free fluoride ions in an amount in the range of from 1 to 500 ppm.
 12. The method according to claim 1, wherein polymer (P) does not contain any phosphonic acid and/or phosphonate groups.
 13. An acidic aqueous composition (A), wherein the acidic aqueous composition (A) has a pH value in a range of from 2.5 to <5.0 and comprises (a) zirconium ions in an amount in a range of from 10 to 200 ppm, calculated as metal, (b) at least one polymer (P), wherein the polymer (P) is selected from the group consisting of poly(meth)acrylic acids and (meth)acrylic copolymers, which bear carboxylic acid groups, and mixtures thereof, and (c) molybdenum ions in an amount in a range of from 0.5 to 10 ppm, calculated as metal, wherein the relative weight ratio of zirconium ions (a) to molybdenum ions (c), in each case calculated as metal, is in a range of from 40:1 to 7.5:1.
 14. A method of using the acidic aqueous composition (A) of claim 13, the method comprising using the acidic aqueous composition(A) for treating at least one surface of a substrate, wherein said surface is at least partially made of at least one metal.
 15. A substrate comprising at least one surface, wherein said surface is at least partially made of at least one metal, wherein said at least one surface has been treated according to the method of claim
 1. 16. The method according to claim 1, wherein the relative weight ratio of zirconium ions (a) to molybdenum ions (c), in each case calculated as metal, is in a range of from 12:5 to 10:1.
 17. The method according to claim 1, wherein the acidic aqueous composition (A) comprises zirconium ions in an amount in a range of from 20 to 35 ppm, in each case calculated as metal, and, molybdenum ions in an amount in a range of from >2 to 3 ppm, in each case calculated as metal.
 18. The method according to claim 1, wherein the acidic aqueous composition (A) has a pH value in a range of from 3.2 to 3.8.
 19. The method according to claim 1, wherein the acidic aqueous composition (A) has a temperature in a range of from 20 to 30° C.
 20. A substrate comprising at least one surface, wherein said surface is at least partially made of at least one metal, wherein said at least one surface has been treated by the acidic composition (A) according to claim
 13. 